1. Field of the Invention
The invention relates to thermal responsive switches and, in particular, to a switch utilizing the thermal expansion and contraction of a gas for operating a pair of contacts.
2. Description of the Prior Art
The prior art, as exemplified in U.S. Pat. Nos. 2,221,633, 2,627,911, 3,282,325, 3,521,814, and 3,568,123, contains many devices and switches which are thermally operated. Some of the thermal responsive devices have enclosed chambers containing an activated material, such as activated charcoal, with a thermally expandable and contractable gas, such as difluorodichloromethane, dimethylether, carbon dioxide, argon or nitrogen. Activated charcoal is made by eroding funnel-like pores or cavities in carbonized organic materials, such as wood, coal, coconut husks, bones, etc. by a reactive material, such as steam, carbon dioxide or the like. While gas expansion thermally operated switches containing an activated charcoal exhibit improved operation due to a larger volume or pressure change per degree of temperature change over switches containing only gas, attempts to manufacture such activated charcoal containing switches in quantities have generally met with failure; it has been impossible to predict or avoid large variations in volume or pressure change per degree of temperature change in different batches of activated charcoal thus producing switches which are actuated at different temperature; the increase in volume or pressure change per degree of temperature change was not sufficiently large to warrant the added manufacturing cost; and the activated charcoal containing switches were substantially deficient in volume or temperature change per degree of temperature change compared to alternate switches using vapor expansion from a liquid, such as mercury, to produce a substantial movement of the contacts making rapid positive opening and closing of the contacts possible.
Also, the prior art, as exemplified in U.S. Pat. Nos. 1,744,735, 3,258,363, 3,442,819, 3,516,791, and the publication (USSR Academy of Sciences, M. M. Dubinin, "Thermal Treatment and Microporous Structure of Carbonaceous Adsorbents", Proceedings of the Fifth Conference on Carbon, Vol, 1, 1962, pages 81-87) contains many adsorbent carbon materials including decomposed polyvinylidene chloride and polyvinylidene fluoride. Adsorbent carbon materials are widely used in removing contaminants or the like from gases or liquids. Polyvinylidene chloride and polyvinylidene fluoride, in particular, have been recognized for their "molecular sieve" properties; that is, their ability to adsorb certain gaseous materials which have small molecular size while being incapable of adsorbing other gaseous materials which have larger molecular sizes.
Many prior art flame switches employ mercury actuators with a mercury containing bulb for sensing a flame. Due to the corrosive properties of mercury, particularly at temperatures associated with flame sensing, such mercury actuators are subject to deterioration and failure. The prior art has failed to produce suitable flame sensing switches which are practical and acceptable replacements for switches employing mercury actuators.